The invention relates to an arrangement for offset compensation of two orthogonal sensor signals S(x) and S(y), which are supplied by two sensors (2, 3) and are preferably designed for angle measurements, the offset compensation taking place in dependence on the geometric arrangement of Free pairs of test values P1(x1, y1), P2(x2, y2) and P3(x3, y3) of the sensor signals S(x) and S(y) in a system of coordinates, the center of a circle on which the three pairs of test values P1(x1, y1), P2(x2, y2) and P3(x3, y3) are situated in the system of coordinates having center coordinates xcex94x and xcex94y relative to die origin of the system of coordinates.
Many known measuring systems determine the test quantity to be measured from two test signals x and y which are mutually orthogonal, i.e. show a mutual phase shift of 90xc2x0. The one measuring channel thus generates a test quantity proportional to the sine of the quantity to be ascertained and the other a signal proportional to the cosine of said quantity. Such an arrangement renders possible, for example, the calculation of an angle in angle measurement systems. A change in the value of the signal amplitude is unimportant here as long as this amplitude changes in an identical fashion with temperature or owing to other influences in both channels, Nevertheless, so-called offsets adversely affect the measuring accuracy. These offsets may be regarded as superimposed DC voltage or current signals, which are to be reduced to a minimum. The offset compensation in the manufacture of the sensors is an intricate matter and is avoided. Instead, it is attempted to carry out an offset compensation in the application surroundings.
U.S. Pat. No. 5,297,063 discloses an arrangement for this purpose which carries out an on-line compensation of the offsets in that the center is calculated of a circle on which three test value pairs are situated. This circle center can be calculated from known geometric relations. A disadvantage of this arrangement is that the procedure is comparatively complicated and that a microprocessor is required.
It is an object of the invention to provide an arrangement with an offset compensation which is as simple as possible.
According to the invention, this object is achieved in that correction means are provided which carry out a sign determination of the values xcex94x and xcex94y of the center coordinates of the circle on which the three pairs of test values P1(x1, y1), P2(x2, y2) and P3(x3, y3) are situated in a repetitive cycle in each measuring cycle, while in each measuring cycle at least one of the pairs of test values differs from the pair of test values used in the preceding measuring cycle, and which means generate correction signals Kx and Ky with which the sensor signals S(x) and S(y) are complemented, in that the correction means in every measuring cycle increase or decrease the offset correction signal Kx by a correction value KWx compared with the offset correction signal Kx of the previous measuring cycle in dependence on the sign of the center coordinate xcex94x , and in that the correction means in every measuring cycle in a similar manner increase or decrease the offset correction signal Ky by a correction value KWy compared with the offset correction signal Ky of the previous measuring cycle in dependence on the sign of the center coordinate xcex94y.
The arrangement according to the invention bases its operation on the geometric relationships, i.e. that the three pairs of test values derived from the test signals S(x) and S(y) lie on a circle whose center characterizes the offsets xcex94x and xcex94y of the two sensor signals S(x) and S(y). The arrangement according to the invention does not carry out an exact calculation of the offsets xcex94x and xcex94y, so as to keep the expenditure as small as possible. Instead, only the signs of the values of xcex94x and xcex94y are determined, which is substantially easier to do and which nevertheless allows for a very good correction. This is achieved in that a correction is carried out in each measuring cycle for three pairs of test values each time, so that the amount of correction required for the values xcex94x and xcex94y becomes smaller at each correction operation and xcex94x and xcex94y are adjusted to as small values as possible, depending on the chosen step size of the change, whereby an optimized offset compensation is obtained.
Correction means are for this purpose provided in the arrangement which carry out a sign determination of the values of xcex94x and xcex94y in a cyclical manner. xcex94x and xcex94y here form the center coordinates of that circle on which the three test value pairs P1(x1, y1), P2(x2, y2) and P3(x3, y3) are situated, which pairs should have been previously determined, and one test value pair at least should be different from the test value pair used in the previous measuring cycle.
The correction means further generate offset correction signals Kx and Ky which are superimposed on the sensor signals S(x) and S(y). The sensor signals S(x) and S(y) are thus corrected by means of the offset correction signals Kx and Ky, so that an offset compensation of these signals is achieved.
This is done in the same manner for the two offset correction signals, but independently of one another. First the said sign of the center coordinate xcex94x is thus determined for the purpose of the offset correction signal Kx in each measuring cycle for the three pairs of test value P1(x1, y1), P2(x2, y2) and P3(x3, y3). Depending on this sign, the offset correction signal kx is then increased or decreased by a correction value KWx, depending on the sign which was determined. The offset correction signal Kx starts from that of the preceding cycle, i.e. the offset correction signal Kx is increased or decreased by the correction value KWx in each cycle in dependence on the value of the center coordinate xcex94x determined in that cycle. In this manner the offset correction signal Kx is increased or decreased in each cycle such that the value of the center coordinate xcex94x decreases continually and approaches a minimum, the value of which depends on the value of the chosen correction value KWx. This is an iterative process in which an offset compensation takes place in several measuring steps, i.e. several measuring cycle periods, which process is close to optimal and nevertheless requires very little expenditure, because a determination of the sign of the values of xcex94x is sufficient.
The procedure for obtaining the offset compensation for the signal S(y) corresponds to the procedure described with reference to the sensor signal S(x); however, the offset compensation for both signals is carried out mutually independently.
The change in the value of the offset correction signal Kx by the correction value KWx applied in each cycle may advantageously be inexpensively provided by means of two integrators. The integrators then merely receive a signal in each cycle indicating whether a change in the offset correction signals by an upward correction value or by a downward correction value is to be applied, whereupon the previously integrated values forming the offset correction signals of the previous cycle are increased or decreased by the new correction value.
The correction values KWx and KWy may have programmable levels, as is provided for in a further embodiment of the invention as defined in claim 3. The value may then be differently programmed, for example, in dependence on the field of application.
If the correction values KWx and KWy have the same value in every cycle, then a correction of the offset compensation can only be applied by that value in each cycle. If a faster offset compensation is desired or required in certain cases, it is advantageous not to choose the values of KWx and KWy to be constant but to make them dependent on the sign found for the two center coordinates xcex94x and xcex94y in the previous cycle and on the correction values KWx and KWy chosen in the previous cycle. This is implemented such that, for example, the correction value KWx is doubled in a measuring cycle n if in the preceding measuring cycle nxe2x88x921 the same sign was determined for the center coordinate xcex94x as in the present cycle n. It is recognized thereby that the offset compensation as regards the sensor signal S(x) has to take place in the same direction, i.e. with the same sign, as in the previous cycle run. The correction value KWx may then be advantageously doubled because it may be assumed that a greater offset correction is required in the same direction. This process will repeat itself until the sign of the coordinate xcex94x changes, whereupon a return is made to the original value of KWx, which may be a fixed or a programmable value. The offset compensation correction process is speeded up as a result of this whenever an offset error of the signal of S(x) with the same sign is detected in several consecutive cycles. Obviously, the procedure for the signal S(y) is the same, again independent of the correction of the signal S(x), i.e. the changes in the values KWx and KWy are applied independently of one another.
To obtain a sufficient accuracy of the center coordinates xcex94x and xcex94y and thus also of the offset compensation, the three pairs of test values P1(x1, y1), P2(x2, y2) and P3(x3, y3) shall fulfill certain geometric requirements which have the result that the determination of the sign of the center coordinates xcex94x and xcex94y is possible with a sufficient accuracy.
According to a further embodiment of the invention, the offset correction signals Kx and Ky may be advantageously limited by means of limiters so that they cannot assume random values. The limiting action prevents the algorithm from diverging owing to signal errors which persist for longer periods.
In combination with the variable correction values KWx and KWy, it may be provided that the correction values KWx and KWy are reset to their initial values when the limit value is reached.
The limit values for the limitation may be set when the arrangement is initialized.
Setting of the start values of the offset Correction signals Kx and Ky may also be carried out during such an initialization.
Other embodiments indicate in concrete terms for the adjustment of the sensor signals S(x) and S(y) in further embodiments of the invention how a sign determination may be carried out on the three pairs of test values P1(x1, y1) P2(x2, y2) and P3(x3, y3) in a comparatively simple manner by means of concrete equations indicated in the claims. A coordinate transform is carried out for determining the sign of the center coordinate xcex94y because in this manner similar equations can be used for determining the sign of xcex94y to those used for determining the sign of xcex94x. It suffices to enter the relevant coordinate values of the pairs of test values in the equations; for the rest the equations are the same for the two center coordinates xcex94x and xcex94y owing to the transform.